The invention relates to a position-sensitive radiation detector provided with a semiconductor structure comprising;
a wafer of semiconductor material of a first conductivity type having two principal surfaces situated at a relatively short distance from each other, the dimensions of which are sufficient to enclose the desired radiation detection surface, PA1 an electrode structure composed of one or more strip-type electrode regions of a second conductivity type in the first principal surface and at least one electrode region of the second conductivity type in the oppositely situated part of the second principal surface, voltages being applied, during operation, to the electrodes of the electrode structure which are such that, on the one hand, the part of the semiconductor wafer between the electrodes of the electrode structure is completely depleted and, on the other hand, a drift field is generated in said depleted part of the structure, PA1 a detection electrode region, of the first conductivity type provided in one of the principal surfaces, a voltage applied to which during operation is such that mobile charge carriers can be attracted by the drift field, PA1 "Silicon detectors" by R. Klanner in Nuclear Instruments and Methods in Physics Research A235 (1985), pages 209-215, PA1 "Semiconductor Drift Chambers for Position and Energy Measurements" by P. Rehak et al. in Nuclear Instruments and Methods in Physics Research A235 (1985), pages 224-234, PA1 "Progress in Semiconductor Drift Detectors" by P. Rehak et al. in Nuclear Instruments and Methods in Physics Research A248 (1986), pages 367-378, and PA1 "Evaluation of Prototype Silicon Drift Detectors" by J. Ellison et al. in Nuclear Instruments and Methods in Physics Research A260 (1987), pages 353-360. PA1 at least one further electrode structure composed of one or more electrode regions of the second conductivity type in the first principal surface and at least one electrode region of the second conductivity type in the oppositely situated part of the second principal surface, voltages being applied, during operation, to the electrodes of the further electrode structure, which voltages are such that, on the one hand, the part of the semiconductor material wafer between the electrodes of the further electrode structure is completely depleted and, on the other hand, a further drift field is generated in said depleted part of the structure, and the further electrode structure being positioned with respect to the first mentioned electrode structure and with respect to the detection electrode region in a manner such that charge carriers originating from the drift field within the first mentioned electrode structure will be intercepted in the drift field within the further electrode structure and will PA1 be able to reach the detection electrode region via said further drift field.
a detector/amplifier circuit connected to the at least one electrode region in the oppositely situated part of the principal surface being designed to emit, during operation, a start signal at the instant at which charge carriers are generated in the depleted part of the structure as a consequence of incident radiation or elementary particles, and a detector/amplifier circuit connected to the detection electrode region being designed to emit, during operation, a stop signal at the instant when charge carriers reach the detection electrode region via the drift field.
A position-sensitive radiation detector of the type described above is disclosed by the U.S. Pat. No. 4,688,067. In said known position-sensitive radiation detector, both the first and the second electrode structures are constructed with the aid of strip-type electrodes which are provided parallel to one another at a short distance from one another in the principal surface concerned of the wafer of semiconductor material. Said known structure is only position-sensitive in one direction, namely in the direction perpendicular to the strip-type electrodes, or in other words, in the direction in which charge carriers which are generated during the detection of radiation will start to move in the drift field which is generated in the wafer of semi-conductor material.
The at least one electrode region mentioned in the second principal surface may be formed by a number of strip-type electrodes as is described in the abovementioned U.S. Pat. No. 4,688,067, but it may also be constructed as a single electrode region such as is described, for example, in the German Offenlegungsschrift 3,415,439. If use is made of a number of strip-type electrodes in the second principal surface, they are preferably capacitively coupled to one another in order to reduce the number of necessary connecting conductors to each of said electrodes for supplying the voltages needed to deplete the semiconductor wafer and to generate the drift field. If use is made of a single electrode region, a capacitive coupling structure does not need to be effected and, in addition, it is possible for a single amplifier to be sufficient to generate the start signal during operation.
If radiation is intercepted or an elementary particle is detected with this known structure, charge carriers will be generated at the position where the radiation is intercepted or where the particle passes through the structure in the wafer of semiconductor material respectively. Depending on the conductivity type used in the semiconductor structure, the charge carriers of the one type will start to move directly in the thickness direction towards one of the electrode regions of the second conductivity type. This charge carrier movement results in the generation of a start signal in a detector/amplifier which is connected to said electrode structure. The charge carriers of the other type will start to move in the drift field approximately parallel to the two principal surfaces until they reach the detection electrode and are attracted by said detection electrode. A stop signal is then generated with the aid of a detector/amplifier which is connected to the detection electrode. The time difference between the start signal and the stop signal is characteristic of the length of the path which the charge carriers concerned have traversed in the drift field and therefore of the position at which the elementary particle was intercepted in the wafer of semiconductor material.
Further details relating to the operation of such a known position-sensitive radiation detector may be found not only in the abovementioned US Pat. No. 4,688,067 and the German Offenlegungsschrift 3,415,439 mentioned, but also in the following articles:
Attempts have already been made to design a position-sensitive radiation detector with two-dimensional position sensitivity. A brief description of an example thereof is to be found in the abovementioned paper entitled "Silicon Detectors" by R.Klanner, in particular FIG. 7 and the associated description on page 212. The detector structure shown in said figure comprises a detection electrode, designated as the anode, which is not constructed as a single strip-type electrode but is constructed as a row of separate anode regions, each connected to its own electronic amplifier circuit. If an elementary particle is intercepted at a certain position in this detector structure or if radiation is intercepted, this will release charge carriers. Depending on the polarity, some of these charge carriers will start to move in the drift field towards the segmented anode or detection electrode. The drift time to the anode is again characteristic of the distance traversed to the anode and is consequently characteristic of the position coordinate. The position of said anode region which emits the stop signal at the end of the drift period is characteristic of the other position coordinate.
A disadvantage of such a structure is that each anode segment requires its own read-out amplifier. With a relatively low resolution and a relatively large length dimension of the anode as a whole, it may be necessary to use a few tens up to even many hundreds of amplifiers to make such a structure function with the desired accuracy.